GB1287505A - Microporous films and their production - Google Patents

Abstract

1287505 Microporous films CELANESE CORP 13 Nov 1970 [13 Nov 1969 28 Oct 1970] 54057/70 Heading B5B [Also in Division C3] An open celled microporous polymer film, of which the pore size is less than 5000 Angstroms, has a crystallinity above 30%, an elastic recovery of at least 50% (from 50% strain at zero recovery time, 25‹ C. and 65% R.H.), a nitrogen flux greater than 35À4 g. mole/cm.<SP>2</SP> min. 10<SP>3</SP> and a bulk density no greater than 95% of that of an equivalent non-porous film. The film is made by stretching in two steps in the same direction a non-porous film of an elastic polymer, e.g. polyethylene, polypropylene or a trioxane-ethylene oxide copolymer, having a crystallinity above 20%. The first stretch is at a temperature below that at which melting begins and produces pore spaces perpendicular to the stretch direction; the second stretch is above the temperature at which melting begins but below the fusion temperature and produces pore spaces elongated in the stretch direction. No more than 5% intermediate shrinkage is allowed between these stretching steps. The film is finally heatset under tension without allowing more than 15% shrinkage or further stretching. The first stretch may be at 10-70‹ C. and the second at 120-150‹ C. Heat setting may be at 80- 160‹ C. The ratio of "first" to "second" stretching amounts may be 0À10 : 1 to 0À99 : 1, the total stretch being 10-300%. The initial film may have been drawn down on extrusion to provide orientation, followed by heat-setting to effect crystallization.